The primary reasoning for this is because of Gygax's study of anthropology.
Priests during the dark ages often favored staves and other blunt objects that could be used more for policing and self defense against other weapons than actual harm. Thus if used properly they would not cause bleeding (directly) but maybe severe bruising or a broken bone.
EDIT 1: p.166 of "The Historical Atlas of Knights and Castles", Dr. Ian Barnes
The 13th Century Mace ... was a cavalry and elite weapon, especially favored by fighting clerics (who would rather crack a skull than spill blood).
Also This article includes
The clergy was forbidden to shed blood, and thus a sword was inhibited, it might have been thought was sufficient to keep them from the battle field. But not so; They adopted the mace; though they could not cut a man's throat, yet they might break his head